1) Field of the Invention
The present invention relates to dispersion particles for a fluid having a characteristic of a magnetic fluid susceptible to a magnetic field and a characteristic of an electrorheological fluid whose viscosity can increase with an applied electric field simultaneously and a fluid used the same, and particularly to a fluid capable of outputting a large force at a high response speed.
2) Prior Art
A magnetic fluid is a colloidal solution, which is a uniform dispersion of ferromagnetic particles in a solvent, and, when a magnet is provided near the magnetic fluid, the entire fluid is attracted towards the magnet and behaves as if the entire fluid is apparently charged with a magnetism.
Furthermore, the magnetic fluid has such a characteristic that a large force can be induced in the magnetic fluid with an applied magnetic field. By virtue of this characteristic, the magnetic fluid is utilized for rotating shaft sealing, and further application to dampers, actuators, gravity separation, ink jet printers, etc. can be expected.
A typical process for preparing a magnetic fluid is a chemical coprecipitation process disclosed in JP-A 51-44579, where an aqueous slurry of magnetic prepared from an aqueous solution of ferrous sulfate and an aqueous solution of ferric sulfate is admixed with a surfactant, followed by water washing, drying and dispersion into an organic solvent, thereby preparing a magnetic fluid.
An electrorheological fluid, on the other hand, is a suspension of inorganic or polymeric particles in an electrically insulating liquid, whose viscosity can be rapidly and reversibly changed from a liquid state to a plastic state or to a solid state or vice versa upon application of an electric field thereto. A high response speed is one of the characteristics.
As dispersion particles, those whose surfaces are readily depolarizable under an electric field are usually used. For example, as inorganic dispersion particles, silica is disclosed in U.S. Pat. No. 3,047,507, British Patent No. 1,076,754 and JP-A 61-44998, and zeolite is disclosed in JP-A 62-95397. As polymeric dispersion particles, arginic acid, glucose having carboxyl groups and glucose having sulfone groups are disclosed in JP-A 51-33783; polyacrylic acid cross-linked with divinylbenzene is disclosed in JP-A 53-3186; and resol-type phenol resin is disclosed in JP-A 58-179259.
As an electrically insulating liquid, mineral oil, silicone oil, fluorohydrocarbon-based oil, halogenated aromatic oil, etc. are known.
It is preferable from the viewpoint of higher electrorheological effect that water is adsorbed on the surfaces of dispersion particles. In most cases, the electrorheological fluid contains a small amount of water.
Mechanism of increase in the viscosity of an electrorheological fluid with an applied electric field can be clarified on the basis of the electric double layer theory. That is, an electric double layer is formed on the surfaces of dispersion particles of an electrorheological fluid, and when there is no application of an electric field, dispersion particles repulse one another on the surfaces and are never in a particle alignment structure. When an electric field is applied thereto, on the other hand, an electrical deviation occurs in the electrical double layers on the surfaces dispersion particles, and the dispersion particles are electrostatically aligned to one another, thereby forming bridges of dispersion particles. Thus, the viscosity of the fluid is increased, and sometimes the fluid is solidified. The water contained in the fluid can promote formation of the electrical double layer.
Application of the electrorheological fluid to engine mounts, shock absorbers, clutches, ink jet printers, etc. can be expected.
However, the magnetic fluid still has such problems that neither high permeability nor higher response speed as aims to a quick response is obtainable. When it is used as a seal, a low sealability is also one of the problems. These problems are obstacles to practical applications. The electrorheological fluid still has such a problem that the torque induced upon application of an electrical field is so small that no larger force can be obtained.